1. Field of the Invention
The present invention relates to a method of forming a contact. More particularly, the present invention relates to a method of forming a contact which includes a step of filling a contact hole with tungsten (W) by chemical vapor deposition (CVD).
2. Description of the Related Art
When a plurality of contact holes which have various different depths formed in an interlayer insulating film are to be filled with a conductive film, a blanket CVD technique is used. According to a prior art, by using known techniques, an impurity diffused region and the like are formed in a surface portion of a silicon substrate, and then an interlayer insulating film covering the substrate is formed. The interlayer insulating film is, for example, an oxide film (e.g., thickness: 16000 .ANG.) formed by CVD. Thereafter, by using photolithography and etching techniques, predetermined portions of the interlayer insulating film are selectively etched, thereby forming contact holes in the interlayer insulating film.
Next, by a reactive sputtering technique, a titanium layer (thickness: about 500 .ANG.) is formed on a surface of the interlayer insulating film so as to cover the inner walls of the contact holes and the exposed portions of the silicon substrate. For example, the sputtering is conducted under such conditions that the substrate temperature is 200.degree. C. and the applied voltage is about 900 volts (V). Then, without changing the above sputtering conditions, a mixed gas containing nitrogen and argon (the flow ratio (nitroGen/argon)=15/85) is introduced into a chamber, and a portion of the titanium layer having a depth of about 1000 .ANG. from the surface thereof, whereby a titanium nitride layer is formed. As a result, a Ti/TiN film is formed. The Ti/TiN film includes a titanium layer having a thickness of 500 .ANG. and a titanium nitride film having a thickness of 1000 .ANG.. The Ti/TiN film is provided for enhancing the adhesion of a tungsten layer to the interlayer insulating film.
Next, a nucleation step is performed by CVD utilizing silane reduction in which WF.sub.6 and SiH.sub.4 (silane) are used as a reaction gas. The ratio of the flow rates is typically set to be about 1. The substrate temperature is set in the range of 450.degree. C. to 475.degree. C. (the wafer temperature is set in the range of about 415.degree. C. to about 440.degree. C.). As the result of the nucleation step, a first tungsten layer (e.g., thickness: about 300 .ANG.) with high crystallinity is formed. The relatively thin first tungsten layer provides growth nuclei for uniformly growing a second tungsten layer in an early growth stage for growing the second tungsten layer by blanket CVD utilizing hydrogen reduction.
After the nucleation step, by the blanket CVD utilizing hydrogen reduction, the second tungsten layer is grown on the first tungsten layer, whereby the contact holes are completely filled with tungsten. Thereafter, the second tungsten layer is etched back from the top face thereof.
According to the above prior art, the obtained contacts can attain a relatively low contact resistance. However, there arises a problem of an increase in leakage current. This is caused by the reaction of unreacted WF.sub.6 which has passed through the Ti/TiN film with silicon of the substrate during the formation of the second tungsten layer by the blanket CVD utilizing hydrogen reduction. By this reaction, the silicon in the surface portion of the substrate is eroded, so that the pn junction of the impurity diffused region is damaged. Because of the damage of the pn-junction, the leakage current at the contacts increases.